Portions of a marine propulsion unit that extend into the water are susceptible to damage by underwater objects. The portions that extend below the hull of a marine vessel are especially susceptible, particularly the leading components such as the nose section and the skeg. A skeg is a fin-shaped feature that extends below the gear case of a marine propulsion unit such as an outboard motor, a stem drive, or a pod unit. The skeg extends below the radius of the propeller (or “prop”) of the marine propulsion unit and serves to protect the propeller from damage from underwater impacts.
A breakaway skeg is designed to break away from the lower portion of the marine propulsion unit when the breakaway skeg impacts an underwater object. The breakaway skeg must be designed to account for impacts from the front, rear, side, and bottom. As the breakaway skeg is attached to the lower portion of the gear case, the attachment features of the breakaway skeg must minimize damage to the lower portion of the gear case when it breaks away. Damage to the lower portion of the gear case can make for a very expensive repair. In addition, excessive damage to the lower portion of the gear case can lead to loss of gear case oil and therefore extensive gear train damage.
The design of the attachment features of the breakaway skeg may account for impacts from the bottom of the marine propulsion unit. A rigidly and strongly-attached skeg would transmit impact forces upward through the marine propulsion unit and into the vessel's hull where the forces could potentially cause hull damage or even a hull breach.
The attachment features of the breakaway skeg may therefore be designed to provide a strong enough attachment to the lower portion of the gear case to stay intact during navigation but must fracture or breakaway above a predetermined stress in order to prevent damage to the lower portion of the gear case.
U.S. Pat. No. 7,435,147 to Eichinger, issued Oct. 14, 2008, entitled “Breakaway skeg for a marine propulsion device,” discloses a breakaway skeg that accounts for an impact force L at locations along the total height of the skeg. The breakaway skeg disclosed by Eichinger, however, does not address impact forces from the bottom that would transmit upward through the marine propulsion unit and into the vessel's hull.